In the 90's I read a book about two children (I think they were siblings, boy and girl) who climbed a tree. At the end they found a door to another world. After some time the (fantasy) worlds switched to another one. Once they got stuck in a world, because the world went on traveling and therefore the door couldn't be reached anymore. They met many curious individuals during their adventures.
It's a completed novel for kids. I've read it in German, but I can't remember if the author was a German one, so it might've been available in English as well. It was a fairly thick book.
EDIT: After some thought I remember a pancake kind of person